Re: [PATCH v5 02/15] cmake: use "-S" and "-B" to specify source and build directories

> Rather than the multi-line "mkdir/cd/cmake" recipe provide an
> equivalent one-liner using the "-S" and "-B" options, and then suggest building with "make -C <build-dir>".
>
> The rest of these instructions discuss e.g. running tests from our
> top-level "t/" directory, so it's more helpful to avoid changing the
> user's current directory.
>
> The "-S" and "-B" options were added in cmake v3.13.0, which is older
> than the version we have a hard dependency on[1].
>
> As an aside The "-p" flag to "mkdir" in the pre-image wasn't needed,
> as "contrib/buildsystems" is tracked